SEMCTL (2)
semaphore control operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#if defined(__GNU_LIBRARY__) && !defined(_SEM_SEMUN_UNDEFINED)
/* union semun is defined by including <sys/sem.h> */
#else
/* according to X/OPEN we have to define it ourselves */
union semun {
int val; /* value for SETVAL */
struct semid_ds *buf; /* buffer for IPC_STAT, IPC_SET */
unsigned short int *array; /* array for GETALL, SETALL */
struct seminfo *__buf; /* buffer for IPC_INFO */
};
#endif
int semctl (int semid ,
int semnum ,
int cmd ,
union semun arg )
DESCRIPTION
The function performs the control operation specified by
cmd
on the semaphore set (or on the
semnum -th
semaphore of the set) identified by
semid .
The first semaphore of the set is indicated by a value
0
for
semnum .
Legal values for
cmd
are
IPC_STAT
Copy info from the semaphore set data structure
into the structure pointed to by
arg .buf \fR.
The argument
semnum
is ignored.
The calling process must have read access privileges on the semaphore set.
IPC_SET
Write the values of some members of the
semid_ds
structure pointed to by
arg .buf
to the semaphore set data structure, updating also its
sem_ctime
member.
Considered members from the user supplied
struct semid_ds
pointed to by
arg .buf
are
B
sem_perm.uid
sem_perm.gid
sem_perm.mode /* only lowest 9-bits */
R
The calling process effective user-ID must be one among super-user,
creator or owner of the semaphore set.
The argument
semnum
is ignored.
IPC_RMID
Remove immediately the semaphore set and its data structures
awakening all waiting processes (with an error return and
errno
set to
EIDRM ).
The calling process effective user-ID must be one among super-user,
creator or owner of the semaphore set.
The argument
semnum
is ignored.
GETALL
Return
semval
for all semaphores of the set into
arg .array \fR.
The argument
semnum
is ignored.
The calling process must have read access privileges on the semaphore set.
GETNCNT
The system call returns the value of
semncnt
for the
semnum -th
semaphore of the set
(i.e. the number of processes waiting for an increase of
semval
for the
semnum -th
semaphore of the set).
The calling process must have read access privileges on the semaphore set.
GETPID
The system call returns the value of
sempid
for the
semnum -th
semaphore of the set
(i.e. the pid of the process that executed the last
semop
call for the
semnum -th
semaphore of the set).
The calling process must have read access privileges on the semaphore set.
GETVAL
The system call returns the value of
semval
for the
semnum -th
semaphore of the set.
The calling process must have read access privileges on the semaphore set.
GETZCNT
The system call returns the value of
semzcnt
for the
semnum -th
semaphore of the set
(i.e. the number of processes waiting for
semval
of the
semnum -th
semaphore of the set to become 0).
The calling process must have read access privileges on the semaphore set.
SETALL
Set
semval
for all semaphores of the set using
arg .array ,
updating also the
sem_ctime
member of the
semid_ds
structure associated to the set.
Undo entries are cleared for altered semaphores in all processes.
Processes sleeping on the wait queue are awakened if some
semval
becomes 0 or increases.
The argument
semnum
is ignored.
The calling process must have alter access privileges on the semaphore set.
SETVAL
Set the value of
semval
to
arg .val
for the
semnum -th
semaphore of the set, updating also the
sem_ctime
member of the
semid_ds
structure associated to the set.
Undo entry is cleared for altered semaphore in all processes.
Processes sleeping on the wait queue are awakened if
semval
becomes 0 or increases.
The calling process must have alter access privileges on the semaphore set.
RETURN VALUE
On fail the system call returns
-1
with
errno
indicating the error.
Otherwise the system call returns a nonnegative value depending on
cmd
as follows:
GETNCNT
GETPID
GETVAL
GETZCNT
ERRORS
For a failing return,
errno
will be set to one among the following values:
EACCES
The calling process has no access permissions needed to execute
cmd .
EFAULT
The address pointed to by
arg .buf
or
arg .array
isn't accessible.
EIDRM
The semaphore set was removed.
EINVAL
Invalid value for
cmd
or
semid .
EPERM
The argument
cmd
has value
IPC_SET
or
IPC_RMID
but the calling process effective user-ID has insufficient
privileges to execute the command.
ERANGE
The argument
cmd
has value
SETALL
or
SETVAL
and the value to which
semval
has to be set (for some semaphore of the set) is less than 0
or greater than the implementation value
SEMVMX .
NOTES
CONFORMING TO
SVr4, SVID. SVr4 documents more error conditions EINVAL and EOVERFLOW.
SEE ALSO
|
